“Flex Seal” at $20 per can is just undercoating which runs about $3 per can.
I could use a compound like that on my boat, if it held up.
It doesn’t appear to be automotive undercoating.
It behaves like liquid neoprene. Smells the same too.
It is a little pricey, and I buy the Home Armour Flexible Sealer for $20, for two cans. It’s not that one has constant uses for it...but its good to have when you need it.
Sometimes the brush on stuff is too heavy for what you want to accomplish.